A rangefinder is used to measure the distance separating it from a target. An optical rangefinder uses the propagation of the light as a measurement means. It comprises a transmitter and a receiver. It transmits light toward the target and detects a fraction of this light returned by the target. The distance is obtained based on the propagation time back and forth of the light from the transmitter to the receiver. The transmission is modulated in time. The transmitted light transports this modulation to the target. The light is absorbed by the atmosphere along the outward path. It is then absorbed and reflected or backscattered by the target and then absorbed by the atmosphere on the return path; it is diluted along the return path by a factor proportional to the square of the distance. A fraction of this returned light transports the modulation to the receiver of the rangefinder. This time modulation makes it possible to identify the start of the pulse and identify its return by the receiver. The elapsed time between these two events makes it possible to calculate the distance between the rangefinder and the target based on the speed of propagation of the light in the media that are passed through.
When the distance increases, the quantity of light detected decreases rapidly. To increase the ranging distance despite these atmospheric losses, the following ways are possible:                increasing the energy per pulse, but this increase is limited by the constraints of ocular safety and by the volume of the transmitter which increases with the energy per pulse,        increasing the dimension of the receiving pupil but this increases the dimensions of the system,        increasing the sensitivity of the receiver with multipulse systems using micro-lasers or fiber optic lasers. This makes it possible to use post-integration. There is increase in the mean power (energy per pulse×rate) without increasing the energy per pulse.        
Currently there are three main categories of laser rangefinders.                Rangefinders having a modulated continuous transmission        Multipulse rangefinders        Monopulse rangefinders        
The rangefinders having a modulated continuous transmission are used with cooperative targets of which the measurement time is not critical. A cooperative target is for example fitted with a back reflector, and therefore returns the light in a narrow cone in the direction of the transmitter. The system is designed so that reception is possible and during transmission.
For noncooperative targets situated at long distances of the order of several tens of km, the rangefinders usually use a single pulse of great energy limited by ocular safety in the conditions of use: the integrated exposure over 10 seconds, for a wavelength of between 1.5 and 1.8 μm, must remain below 10 000 J/m2. This limit, depending on the applications, allows energies per pulse of from a few millijoules to several tens of millijoules. To achieve good distance accuracy, the pulses have a very short duration: of the order of 10 ns. Detection of the echoes is not possible during the transmission of the pulses.
For short distances (<10 km), it is possible to use laser diodes as a transmitter. The energy per pulse is very low. The performance is obtained by multiple pulses with detection with post-integration. The pulse duration of the order of 10 to 50 ns is very low compared with the period between the pulses which is of the order of 1 to 50 μs. During transmission, reception is blind. The diffusion of the transmitted light by the atmosphere over a short distance (from a few meters to a few tens of meters) blinds reception. Beyond this, detection takes place during the period between the pulses. Detection of the echo is a detection of energy.
Post-integration has certain drawbacks.
Specifically, note that:
if, for a transmitted pulse, there is for the echo a signal-to-noise ratio S/B,
therefore n transmitted pulses gives (nS)/(n1/2 B), or (n1/2 S)/B, hence an improvement of factor n1/2.
But in the case of post-integration, the frequency of repetition of the pulses (or rate) limits the distance that can be achieved because of the ambiguity concerning the distance. This ambiguity occurs when a detected pulse originates either from the last transmitted pulse, returned by a close target, or from a pulse transmitted earlier and returned by a distant target, without it being possible to determine between these 2 alternatives which target is measured. By accepting a larger timescale of blind reception, each pulse can be replaced by a pulse train.
An optical system for high bit-rate communication in free space also comprises a laser transmitting device for an optical signal and if the communication is two-way, it also comprises a device for receiving the optical signals transmitted by another communication system. The optical signal transmitted is a rapid succession of pulses at a period of repetition typically between 1 ns and 20 ms. The gaps between the pulses have periods similar to the pulse widths. Digital data consist of 0s and 1s. Each data bit is associated with a unitary period: a pulse during this unitary period represents a 1, no pulse during this period represents a 0. The data sequences are also usually encoded by successions of pulses and by periods between the pulses. The peak power of the communication pulses is on average double the mean power of the communication transmission. The transmission is of the modulated continuous type on two levels 0 and 1. In the rest of the description, such a succession of pulses modulated in this way is called the optical communication signal. Several examples (16 examples) of high bit-rate communication signals are shown in FIG. 3b. Over the first 10 ns, the example of the 4th channel corresponds to the following digital sequence: 00100110010.
The transmitting device of the rangefinder and that of the communication system therefore obey contradictory constraints, and their receiving devices. Hence the use of two independent devices for performing the functions of long-range ranging and of high bit-rate optical communication in free space.
Such apparatus are then bulky and heavy. The object of the invention is to alleviate these drawbacks.